Not applicable.
1. Field of the Invention
This invention relates to a method for converting methoxy compounds, such as methanol and dimethyl ether, into olefins, preferably ethylene, by contact with catalyst specifically prepared and/or conditioned to achieve a high yield of ethylene from the quantity of methoxy compound converted.
2. Description of the Related Art
Ethylene today is typically produced by steam cracking of a saturated hydrocarbon, such as ethane and other higher hydrocarbons or mixtures ranging from propane to naphtha and even vacuum gas oils. Ethane is a preferred feedstock since for the quantity of converted carbon the yield of ethylene is significantly higher than that of other higher olefins. Of the olefins produced by steam cracking of a saturated hydrocarbon feedstock ethylene is considered the olefin of greatest market value and hence originates the desire to maximize its production by comparison to propylene or other higher olefins.
Ethane is often available as a feedstock source only in the United States and even then its quantity is limited, and this forces foreign producers and even some U.S. producers into utilization of alternative hydrocarbon feedstocks.
This then has galvanized a search over the last 20-30 years for a viable alternative procedure for production of ethylene. In the 1970s Mobil developed a catalyst which is capable of converting methoxy compounds such as methanol and/or dimethyl ether into olefins (a MTO catalyst) and also into normally liquid saturated hydrocarbons (a MTG catalyst). The catalyst which Mobil developed is a zeolite and the most prevalently used zeolite catalyst composition has been ZSM-5. This catalyst consists of alumina and silica. Mobil describes the use of such ZSM-5 catalyst in both fixed and fluid beds for the conversion of methoxy compounds into synthetic gasoline (methanol to gasoline; MTG). Around 1980 Union Carbide researchers succeeded in building into this molecular sieve catalyst structure various amounts of phosphorus oxides. The resulting molecular sieve catalysts were called SAPO""s, which letters indicate three components of these new zeolite catalysts, namely silica, alumina and phosphorus oxides. See U.S. Pat. No. 4,524,234.
Of the SAPO zeolite catalysts one specific version thereof called SAPO-34 was found to be especially effective for conversion of methanol into olefin mixtures consisting of ethylene, propylene and butylenes. Since then significant attention has been given to the possibility of producing olefins, especially ethylene, from methanol by the use of such methanol-to-olefin (MTO) catalysts.
Initial experiments with the SAPO-34 catalyst were conducted in a fixed bed reactor which produced a 100% conversion of methanol and on a converted carbon basis a yield pattern of about 50% ethylene, 30% propylene and 8% butylene with the remainder of converted carbon going to by-products and coke. See, Methanol to Olefins Process Using Silicoaluminophosphate Catalyst, presented by: Dr. Jeffery M. O. Lewis and Giacomo Corvini of Union Carbide Corporation, copyright 1988. However, quite early in the review of the possibility of using a SAPO-34 catalyst it was decided that due to coking a fixed bed operations was not attractive for commercial processing and attention switched to the possible use of this catalyst in a fluid bed operation. Use of the SAPO-34 catalyst in a fluid bed operation finally resulted in a yield pattern on a converted carbon basis of about 48% ethylene, 33% propylene, and 10% butylene. See xe2x80x9cGas to Olefins Using the New UOP/Hydro MTO Processxe2x80x9d by B. V. Vora, T. L. Marker, P. T. Barger, and H. E. Fullerton of UPO, and H. R. Nilsen, S. Kvisle and T. Fuglerud of Norsk Hydro as presented to Gas Processors Association GCC Chapter, Bahrain, Nov. 22, 1995. More recently in U.S. Pat. No. 5,817,906 a yield of 53.8% ethylene 29.1% propylene and 7.8% butylene has been reported. The UOP articles reporting on this study indicate that a fluidized bed operation utilizing the SAPO-34 catalyst for production of ethylene may have an advantage over naphtha cracking, but ethane steam cracking. was admitted to still be slightly more attractive. This continued preference for ethane steam cracking has been underscored by the fact that in 1997 Union Carbide itself announced their decision to have three new ethylene plants constructed worldwide on the basis of ethane cracking. Although many researchers have tried to arrive at higher ethylene yields in a process utilizing SAPO-34 for the conversion of methanol to ethylene, these efforts have not resulted in an improved process for methanol to olefin conversion.
Accordingly, there is no commercial operation practice today that uses methanol as a feedstock for production of ethylene. This is due to many factors, such as the apparently insurmountable cap upon the yield of ethylene that can be obtained (about 50% maximum reported today) coupled with the rapid coking of a SAPO-34 catalyst which requires its employment in a fluidized bed operation since the frequency of catalyst regeneration that would appear to be necessary for a fixed bed operation appears unacceptable. See U.S. Pat. Nos. 5,817,906 and 5,095,163. Further, the reports on operations with a SAPO-34 catalyst appear to be unanimous on the fact that fresh or freshly regenerated catalyst initially gives poor yields (of ethylene), but on aging of such catalysts the yields improve markedly. This, too, mitigates against the apparent desirability of utilizing a SAPO-34 catalyst in a fixed bed operation for conversion of methanol to ethylene.
Although still a desire of the art, as yet no method of operation with a zeolite catalyst for conversion of methanol to ethylene in a high yield, greater than about 50% of the methoxy carbon content converted to ethylene, has heretofore been found.
This invention comprises an operating method by which a zeolite catalyst for methanol to olefin conversion (a MTO catalyst) can be conditioned and used in a fixed structure mode for the conversion of methoxy compound(s) to ethylene at a very high yield of ethylene for the quantity of methoxy compound(s) reacted. The method of this invention will also minimize the quantities of propylene and butylenes produced and prolong the service time of the MTO catalyst before any regeneration is required.
A principal concept of this invention is that of an intentional balancing of the activity of an MTO catalyst particle against the diffusivity of that catalyst particle so that with a fixed mass comprising a plurality of such MTO catalyst particles, within a range of weight hourly space velocity (WHSV) practical for feeding a methoxy compound through a fixed structure comprising particles of such catalyst, production of ethylene is maximized while production of higher molecular weight olefins, such as propylene and/or butylenes, is minimized; all of this preferably while achieving a maximum conversion of such methoxy composition (70-99%) to ethylene as is consistent with these goals.
So far those materials which have been described in the art as active for catalyzing the conversion of methanol and/or dimethyl ether into olefinic hydrocarbon structures are zeolites. Such zeolites are porous or channeled structures the inlet pores of which are of a definitive range of cross-section size measurable in Angstrom units (xc3x85). These pores and the rest of the internal surface area of the catalyst, together with the exterior boundary that defines the catalyst particle, presents a contactable surface area that has sites of atomic structure that are active points for conversion of methoxy compounds into olefinic hydrocarbon structures with a co-production of water (i.e., 2 CH3OHxe2x86x92C2H4+2 H2O) and also for conversion of ethylene into higher molecular weight olefinic hydrocarbons (i.e., 3 C2H4xe2x86x922 C3H6). Such conversion of methoxy compounds and/or ethylene into other hydrocarbon structures as occurs in contact with an MTO catalyst composition occurs by reason of an appropriate contact of the reactant molecular species with a catalytically reactive site. The surface area presented by the inside surface area of such MTO catalyst compositions (hereafter inside surface area, or ISA) very greatly exceeds that surface area presented by the exterior boundaries that define a particle of such catalyst (hereafter exterior surface area, or ESA). Hence, the number of ISA active catalytic sites existing within the MTO catalyst particles greatly exceeds the number of ESA active sites existing at the boundaries of such catalyst particles.
The transport of mass into contact with the exterior surface areas (ESA) as compared to that of into contact with the inside surface area (ISA) of a catalyst particle therefore does not influence the degree and type of conversion of a methoxy reactant and also of an ethylene reactant. For all practical purposes only the ISA is the effective area of the catalyst for purposes of methoxy conversion.
Through a fixed collective structure of a multiple of such catalyst particles the amount of mass flow that can be achieved per unit of time is primarily dictated by the pressure drop between the inlet and the outlet to such structure that can practically be tolerated and this, in turn, is primarily a function of the interstitial spacing that exists between the packing of such catalyst particles in their so-fixed structure. This maximum mass flow, or any lower value therefrom, then dictates in major part the mass flow which contacts the collective exterior boundaries of the particle collective as compared to the mass that contacts the inside surface area of the particle collective, which is a function of the xe2x80x9cdiffusivityxe2x80x9d of the feed through the inlet pores of the catalyst particles.
The diffusivity, or timed transport of mass, through the pores of a catalyst particle occurs at a much slower rate than that of the flow of mass across the exterior boundary of a catalyst particle; this because the cross-section area of a pore of the catalyst particle is much less than the interstitial spacing that exists between catalyst particles. This then means that per unit of time the degree of conversion of methanol within the catalyst particle is much greater than the degree of methanol conversion that occurs at the surface boundary of a catalyst particle. Such methanol which enters a pore inlet of a catalyst particle can become essentially completely reacted to ethylene while within the catalyst particle before this mass flow begins to approach the outlet pore of that catalyst particle. Thus, at that point within the inside surface of a catalyst particle wherein methanol has become essentially completely depleted because of its conversion to ethylene, thereafter the only mass flow toward the outlet pores is olefinic and there exists only olefins for occupancy of the catalytically active cites which remain inside the catalyst particle. This then creates a condition for the conversion of ethylene into higher olefins, such as propylene (i.e. 3 C2H4xe2x86x922 C3H6).